Composite articles having a plurality of layers, especially those used to make wound dressings, garments, and industrial composites, could be vastly improved if there was a method for adhering the substrate to the foam layer while still maintaining the malleability and characteristics, e.g., biocompatibility, of the original foam. For example, it has been difficult to construct safe, durable wound dressings. Moreover, industrial composite articles with increased durability, malleability and strength have been sought for many years. What is needed is a method of adhering the substrate to the polymer foam without adversely affecting the desired properties of the original untreated foam or substrate, e.g., biocompatibility or moisture vapor transfer rate.
Methods for achieving adhesion between layers of a composite material typically involve a combination of surface modification techniques. Several types of surface modification exist. One type of surface modification is to bind covalently a modifier to a surface of a substrate material. This binding can be achieved in many different ways, such as chemical grafting onto the surface of the substrate through condensation or high energy addition reactions, or oxidizing the substrate away leaving a covalently bound modified surface. Covalently bound modifiers are usually the most durable surface modifications. However, such techniques are complicated, expensive and often environmentally hazardous to employ. Another type of surface modification is to cause an association or entrapment of the modifying molecule (or part of the molecule) with the substrate material. This commingling of modifier molecules and substrate relies on molecular attractions, such as van der Waals forces, dipole-dipole interactions, hydrogen bonding, as well as steric factors to hold the modifier in/on the surface of the substrate. The factors that effect this type of reaction are similar to those that effect thermosol dyeing or blooming. Still another type of surface modification involves the retention of modifier by substrate with only adhesive and cohesive forces between the modifier to the substrate and the modifier to itself, respectively.
Methods for improving adhesion between substrate and polymer layers have previously come at the expense of other qualities, such as durability, malleability, the environment, or performance characteristics. Addressing one aspect of desired qualities usually results in sacrificing other qualities. Conventional treatments for improving adhesion between substrate and polymer layers are typically unable to solve this dilemma and fall into the general categories of (i) surface coatings; (ii) saturations or impregnations; (iii) layers of fibers and/or polymers; (iv) unique chemical compositions; and (v) combinations of the foregoing.
Coatings can be one-sided or two-sided, but tend to be step gradients from one surface through the width of the substrate being treated, as opposed to homogeneous materials or continuous gradients. A step gradient has certain intrinsic disadvantages, due mostly to the fact that the coating composition contacts the substrate at one surface, thereby causing a substrate/coating-composition interface. Adhesion at the substrate/coating-composition interface derives mostly from surface forces, less than optimal mechanical interlocking, and sometimes little to no contribution from the cohesive strength of the modifying or coating material. Secondly, because of the disparate materials plied together, the resultant tactile properties of the composite (e.g., hand, drape) are usually distinctly different than the base fabric. Typically, this interface tends to separate upon prolonged exposure to moisture or upon high stress conditions.
Several references describe laminates or layers of fabrics and/or polymers. Laminations use an adhesive tie coat to keep a film in contact with the fabric surface. This technique exhibits the same limitations described above for coatings, as well as environmental issues with the adhesives and any other part of the film preparation process. Additional difficulties are encountered in ensuring that the mechanical performance differential between the substrate, adhesive, and film is balanced. For example, if shrinkage of any of the three materials passes the initial yield stress of either of the other materials, there will be deformation; further, if it passes the ultimate tensile strength, there will be delamination of the composite. U.S. Pat. Nos. 4,872,220; 5,024,594; 5,180,585; 5,335,372; and 5,391,423; describe articles that use layers of fabrics and/or polymers to prevent blood, microbes, and viruses from penetrating through the fabrics. Similarly, U.S. Pat. No. 4,991,232 describes a medical garment comprising a plurality of plies to prevent blood from penetrating through the garment. Layers of fabrics and/or polymers traditionally result in heavier garments and utilize additional raw materials. Moreover, the coating of a polymer upon a web which has been treated by the above techniques, exhibits the same limitations as discussed above.
Some layering techniques, particularly related to industrial composites, require a combination of steps to improve adhesion and/or obtain multiple layers of polymer and fabrics while shaping the article. U.S. Pat. No. 3,762,978 describes a process for preparing a surface of a cured silicone polymer with a mineral acid for the purpose of adhering another uncured silicone polymer composition. This technique requires specific compositions and is not universally available for all compositions. Moreover, the technique relies on an initial silicone polymer coating, having the limitations of coatings discussed above.
Most industry efforts to improve adhesion of one layer to another have focused primarily on the chemistry involved in adhesion. Many patents, too numerous to cite, involve unique chemical compositions; each composition has unique abilities and limitations related to adhesion, durability and/or other performance characteristics. A sampling of patents related to unique compositions for improved adhesion comprise U.S. Pat. Nos. 4,681,808; 5,292,586; 5,360,852; 5,416,144; 5,374,485; 5,342,870; 4,525,400; 4,483,973; 5,308,887; 5,190,827; 5,175,058; 5,175,057; 5,128,394; 5,096,981; 5,028,485; 4,988,779; 4,794,192; 5,436,303; 5,399,614; 5,714,265; 4,918,126; 4,205,559; 5,023,288; 4,942,093, 5,503,940, 5,700,532; European Publication No. 0 491 483 A1; and an article by Stein et al., in Macromolecules, 19: 2291-2294, 1986. Most of these references identify specific chemical moieties related to adhesion. Two such patents, U.S. Pat. Nos. 5,714,265 and 5,700,532 describe the use of two different curing agents, a platinum-based catalyst polymer and a peroxide-based catalyst polymer, interacting to create an improved adhesion.
What is needed in the art is a single method for adhering the substrate to the polymer foam while preserving the durability, malleability, and characteristics of the substrate and polymer foam.